The present invention relates to an optical transmission system including relay stations and, in particular, to an optical output interrupting system for interrupting an optical output of an optical amplifier in view of safety when detachment of a connector or cutting of an optical cable occurs.
In recent years, the long-distance optical transmission can be achieved by arranging a optical amplifier, which directly amplifies a light signal in the form of light using the erbium doped fiber, In a relay station so as to amplify the light signal which has been attenuated due to transmission loss. Since an optical output amplified by the optical amplifier is very strong in light intensity, it is very dangerous if impinging upon the eye of a workman. Hence, consideration is required to the safety. In particular, when an output connector of the optical amplifier is detached or an optical cable being a transmission line is cut, an immediate interruption of the optical output is required so as to prevent the optical output from adversely affecting a human body. In view of this, an apparatus has been developed that automatically interrupts the optical output by detecting the Fresnel reflection which occurs at an end face of the fiber when the connector is detached or the optical cable is cut.
On the other hand, there has also been a system, wherein up and down optical cables are individually arranged between first and second opposite stations. When the first station detects no arrival of a light signal from the second station through, for example, the up cable due to the cable cutting or the like, the first station stops an optical output to the second station through the down cable so that the second station being a transmitter station of the up cable detects no arrival of a light signal from the first station being a receiver station of the up cable and stops sending the light signal into the up cable which is subjected to the cable cutting or the like. Thus, in this system, no arrival of the light signal should be detected twice until the optical output into the optical cable subjected to the cable cutting or the like is stopped. Accordingly, a relatively long time is required from the occurrence of the cable cutting or the like to the interruption of the optical output.
Japanese First (unexamined) Patent Publication No. H04-313706 discloses an optical output interrupting system, wherein one optical cable is used for up and down bilateral transmissions so that an optical output can be interrupted in a short time from an occurrence of the cable cutting or the like. In this system, when no arrival of the light signal is detected at each of stations arranged at opposite ends of the bilateral transmission optical cable, transmission of the light signal to the opposite station is stopped. Since no arrival of the light signal is detected at the opposite stations substantially at the same time, the optical output can be interrupted in a short time.
On the other hand, Japanese First (unexamined) Patent Publication No. H03-94529 discloses an optical output interrupting system, wherein digital data is optically transmitted in frame structure, and an abnormal state of a transmission line is detected using data at an overhead portion of a frame, and wherein an occurrence of the abnormal state is notified to an opposite station so that the optical output from the opposite station is interrupted.
However, in the system which detects the cable cutting or the like based on the Fresnel reflection, although the detachment of the output connector of the optical amplifier or the cutting of the optical cable at a position near the optical amplifier can be detected, the cable cutting at a place remote from the optical amplifier can not be detected. Further, if dust is adhered to the end face of the fiber, it is possible that an occurrence of the cable cutting can not be detected.
On the other hand, in the system where the receiver station detects no arrival of the light signal and notifies it to the transmitter station so as to interrupt the optical output, even the cable cutting at the position remote from the optical amplifier can be reliably detected. However, when no arrival of the light signal is detected, the receiver station notifies it to the opposite station by stopping the optical output to the opposite station. Thus, even if the up and down optical cables are individually arranged, the cutting of one of the cables renders the communication in both directions impossible. On the other hand, in the system where one optical cable is used for the up and down bilateral transmissions, although no notification is performed, the up and down communications are both disabled at once.
When using the data at the overhead portion, the communication in a direction not subjected to the cable cutting can be ensured. However, it is not preferable to control the overhead portion data at the relay station in the long-distance optical transmission system since this lowers the flexibility or generality of the communication system. Further, there is another problem that a complicated apparatus should be provided per relay station for analyzing the frame to control the overhead portion data.